As the wheel supporting rolling bearing unit, structures shown in FIGS. 10 and 11 are set forth in JP-A-2001-221243, for example. First, a structure of a first example shown in FIG. 10 will be explained hereunder. A wheel 1 constituting the wheel is supported rotatably on an end portion of an axle 3 constituting a suspension system by a wheel supporting rolling bearing unit 2. More particularly, inner rings 5, 5 as a stationary side raceway ring, which constitutes the wheel supporting rolling bearing unit 2, are fitted onto a supporting shaft 4 fixed to the end portion of the axle 3, and then the inner rings 5, 5 are fixed with a nut 6. Meanwhile, the wheel 1 is coupled/fixed to a hub 7 as a rotary side raceway ring, which constitutes the wheel supporting rolling bearing unit 2, by a plurality of stud bolts 8, 8 and nuts 9, 9.
Double row outer ring raceways 10a, 10b that act as a rotary side raceway surface respectively are formed on an inner peripheral surface of the hub 7, and a fitting flange 11 is formed on an outer peripheral surface of the same. The wheel 1 as well as a drum 12 constituting a baking system is coupled/fixed to a one-side surface (an outside surface in the first example, a left side surface in FIGS. 10 and 11) of the fitting flange 11 by the stud bolts 8, 8 and nuts 9, 9.
Balls 14, 14 are provided rollably between the outer ring raceways 10a, 10b and inner ring raceways 13, 13, which are formed on outer peripheral surfaces of the inner rings 5, 5 to act as the stationary side raceway surface respectively, every plural pieces in a state that these balls 14, 14 are held in cages 15, 15 respectively. A double row angular contact ball bearing having a back-to-back arrangement is constructed by combining respective constituent members mutually in this manner, so that the hub 7 is supported rotatably around the inner rings 5, 5 to bear the radial load and the thrust load. In this case, seal rings 16a, 16b are provided between inner peripheral surfaces of both end portions of the hub 7 and outer peripheral surfaces of end portions of the inner rings 5, 5 to isolate a space in which the balls 14, 14 are provided from the external space. In addition, an opening portion of an outer end (Here, an “outside in the axial direction” means the outside of the hub in the width direction when such hub is fitted to the vehicle. Similarly, an “inside” means the inside of the hub on the middle side in the width direction.) of the hub 7 is covered with a cap 17.
Upon using the above wheel supporting rolling bearing unit 2, as shown in FIG. 10, the supporting shaft 4 onto which the inner rings 5, 5 are fitted and fixed is fixed to the axle 3 and also the wheel 1, to which a tire (not shown) is fitted, and the drum 12 are fixed to the fitting flange 11 of the hub 7. Also, a braking drum brake is constructed by assembling in combination the drum 12, a wheel cylinder and shoes (not shown) supported on a backing plate 18 fixed to the end portion of the axle 3. Upon braking, a pair of shoes provided to the inner diameter side of the drum 12 are pushed against an inner peripheral surface of the drum 12.
Next, a structure of a second example shown in FIG. 11 in the prior art will be explained hereunder. In the case of this wheel supporting rolling bearing unit 2a, a hub 7a as the rotary side raceway ring is supported rotatably by a plurality of balls 14, 14 on the inner diameter side of an outer ring 19 as the stationary side raceway ring. Therefore, the double row outer ring raceways 10a, 10b as the stationary side raceway surface are provided on the inner peripheral surface of the outer ring 19 respectively, and first and second inner ring raceways 20, 21 as the rotary side raceway surface are provided on the outer peripheral surface of the hub 7a respectively.
The hub 7a is constructed by using a hub main body 22 as a main shaft member and an inner ring 23 in combination. A fitting flange 11a for supporting the wheel is provided to an outer end portion of the outer peripheral surface of the hub main body 22, and the first inner ring raceway 20 is provided to an middle portion thereof, and also a small-diameter stepped portion 24 that is smaller in diameter than a portion in which the first inner ring raceway 20 is formed is provided to the middle portion thereof near an inner end. Then, the inner ring 23, to an outer peripheral surface of which the second inner ring raceway 21 having a circular-arc sectional shape is provided, is fitted onto the small-diameter stepped portion 24. In addition, an inner end surface of the inner ring 23 is pressed with a caulking portion 25 that is formed by elastically deforming an inner end surface of the hub main body 22 outward in the radial direction. Thus, the inner ring 23 is fixed to the hub main body 22. Further, seal rings 16c, 16d are provided between an inner peripheral surface of the outer ring 19 on both end portions and the outer peripheral surface of the middle portion of the hub 7a and the outer peripheral surface of the inner end portion of the inner ring 23 respectively. Thus, the spaces in which the balls 14, 14 are provided are isolated from the external space between the inner peripheral surface of the outer ring 19 and the outer peripheral surface of the hub 7a. 
In this case, in the case of the above wheel supporting rolling bearing unit 2a shown in FIG. 11, the rigidity can be enhanced because the first inner ring raceway 20 is formed directly on the outer peripheral surface of the middle portion of the hub main body 22. In other words, the first inner ring raceway to be provided in the middle portion of the wheel supporting rolling bearing unit can be formed on the outer peripheral surface of the inner ring prepared as a separate body of the hub main body, and then this inner ring can be fitted/fixed onto the hub main body. In this case, unless an inference of the inner ring into the hub main body is increased, the rigidity is lowered, like a structure shown in FIG. 11, rather than the case where the first inner ring raceway 20 is formed directly on the outer peripheral surface of the middle portion of the hub main body 22. A working of fitting the inner ring as the separate body onto the hub main body from the inner end portion to the middle portion while keeping a large inference is troublesome. In contrast, as shown in FIG. 11, the wheel supporting rolling bearing unit 2a having the high rigidity can be constructed without trouble by employing the structure in which the first inner ring raceway 20 is formed directly on the outer peripheral surface of the middle portion of the hub main body 22.
In the case of the wheel supporting rolling bearing unit 2 (or 2a) as set forth in above JP-A-2001-221243, it is unavoidable that a torque (running resistance of the wheel supporting rolling bearing unit) required to turn the hub 7 (or 7a) is increased because of the presence of the seal rings 16a, 16b (or 16c, 16d) that close the opening portions on both ends of the space in which the balls 14, 14 are provided. As a result, since the running performances, mainly the acceleration performance and the fuel consumption performance, of the vehicle into which this wheel supporting rolling bearing unit is incorporated are worsened, improvement in the running performances is desired in view of the recent trend toward the energy saving. The technology to reduce the sliding resistance between the seal member and the sliding portion of the counter member by mixing plastic fine grains which are impregnated with the lubricant into the rubber composition constituting the seal member is known in JP-A-8-319379. However, no description is given in JP-A-8-319379 that suggests getting of the high-performance structure as a whole by applying the above rubber composition to the wheel supporting rolling bearing unit.
In addition, as the structure that reduced the running torque of the rolling bearing unit by reducing the resistance of the seal-ring providing portion, improvement in the inference of the seal lip as set forth in JP-A-10-252762 was considered in the prior art.
In the case of the wheel supporting rolling bearing unit as the subject of the present invention, even though the running torque should be reduced, the structure capable of keeping the wheel supporting rigidity to ensure the controllability and also preventing sufficiently the entering of the foreign matter into the internal space of the rolling bearing unit to ensure the durability of the rolling bearing unit is needed. In other words, the supporting rigidity must be assured by enhancing the rigidity of the rolling bearing unit to ensure the controllability, nevertheless the rolling resistance of respective rolling members is increased if a preload applied to respective rolling members is increased simply to enhance the rigidity, whereby the running torque cannot be reduced. Also, in case it is considered only that the sliding resistance of the seal ring is lowered simply, prevention of the entering of the foreign matter into the internal space of the rolling bearing unit cannot be sufficiently attained and thus the durability cannot be sufficiently assured.
A wheel supporting rolling bearing unit of the present invention has been made in view of such circumstances, and realizes a structure that has a high rigidity, an excellent durability, and a low running torque.